When a high pass filter is needed on the input to an operational amplifier, a bias resistor, used to bias the input to the operational amplifier, also serves as the resistive component of a high pass RC filter. The capacitive component of the RC filter may be the circuit capacitance of the circuit connected to the input to the operational amplifier, and therefore, may be very small. When the capacitance is very small, and the high pass filter needs to have a very low cutoff frequency, the bias resistor needs to be of a very large value, typically in the 1 gigohm range (10.sup.9 ohms). Obtaining gigohm resistance values within an integrated circuit requires a very large amount of space within the integrated circuit.
Prior art devices, such as those shown in FIGS. 1 and 2, described below, typically use bootstrapping to create an effective large resistance. Using the technique of bootstrapping, the effective resistance of the bias resistor is multiplied by a ratio of two resistors connected between the inverting input of the operational amplifier and the voltage source, and since this ratio may typically have a value of 50, only a 40 megohm resistor is needed for the bias resistor to accomplish an effective resistance of 1 gigohm. The bootstrap ratio is limited by the offset voltage of the amplifier, and creating a 40 megohm resistor requires considerable amount of integrated circuit space.
A FET may be used to create the resistor, however, this method requires that the FET be of a very large size. Because of the large size, there is a large parasitic capacitance within the FET which acts as a voltage divider, reducing the value of the input signal.
There is need in the art then for a circuit that will provide an equivalent high value bias resistance while requiring far less space than that occupied by creating a resistor within an integrated circuit, and can offset the effect of large parasitic capacitance within a large FET. The present invention meets these and other needs.